1. Field of the Invention
This invention relates to the immobilization of microorganisms. More particularly, it relates to a method of immobilizing microorganisms in which living cells of a microorganism are adsorbed on a water-insoluble polymeric carrier and then grown.
2. Description of the Prior Art
In recent years, many attempts have been made to produce useful substances through enzyme reactions using so-called immobilized microorganisms (i.e., an immobilized form of microorganisms having an enzyme activity). The use of immobilized microorganisms has a great advantage in that the enzyme reactions can be carried out in a continuous manner.
The currently known techniques for immobilizing microorganisms in a viable state include:
(1) the method of entrapping microbial cells in the matrix of a polymer gel; PA1 (2) the method of causing microbial cells to be ionically or physically adsorbed on a water-insoluble carrier; PA1 (3) the method of coating microbial cells with a polymer film having semipermeability;
and the like.
Among them, the method (1) of entrapping microbial cells in the matrix of a polymer gel has been investigated to a full extent. By way of example, immobilized microorganism gel beads can be prepared either by suspending microbial cells in an aqueous solution of sodium alginate and then adding the resulting suspension dropwise to an aqueous solution of calcium chloride, or by suspending microbial cells in an aqueous solution of .kappa.-carrageenin and then adding the resulting suspension in an aqueous solution of potassium chloride. Moreover, a mass of immobilized microorganism gel can be prepared by adding acrylamide, N,N'-methylenebisacrylamide, and a polymerization initiator to a suspension of microbial cells and then polymerizing the resulting mixture. These immobilized microorganism gels are characterized in that the catalytic activity for inherent enzyme reactions can be remarkably enhanced by passing a nutrient medium for the cultivation of the microorganism through the gel and thus increasing the number of microbial cells present therein. Although the aforesaid gelation procedures can be aseptically performed on a laboratory scale, very complicated and uneconomical operations are required when they are carried out in large-scale industrial equipment. Moreover, the resulting immobilized microorganism gel may be contaminated with miscellaneous microorganisms, so that undesired enzyme reactions take place concurrently to impair the purity of the desired useful product.
In the method (2) of causing microbial cells to be ionically or physically adsorbed on a water-insoluble carrier, various materials such as clay, silica gel, ion exchange resins, and the like are known to be suitable for use as the carrier. However, this method suffers from the disadvantage that the number of microbial cells adsorbed on the carrier is rather small. Even after the microorganism is grown by passing a nutrient medium through the carrier, the cell concentration is lower than that achieved by the method (1). Nevertheless, the method (2) has the advantage of permitting microorganisms to be aseptically immobilized with comparative ease in industrial equipment.
The method (3) of coating microbial cells with a polymer film having semipermeability is the so-called microencapsulation method. From an industrial point of view, this method is not satisfactory because the microbial cells may be inactivated by the organic solvent or monomer used for the formation of a film and because the complicated procedure for the preparation of microcapsules makes this method expensive.
As described above, the well-known techniques for immobilizing microorganisms can exhibit their advantages in case of enzyme reactions on a small scale. However, when they are utilized on an industrial scale, their disadvantages are so great that it may often be more economical to use free (i.e., not immobilized) microorganisms according to conventional procedure.